Background prior art relating to the use of holographic techniques, in particular kinoforms (phase only holograms), in telecommunications can be found in: U.S. Pat. No. 5,617,227; U.S. Pat. No. 5,416,616; WO03/021341; U.S. Pat. No. 7,457,547; U.S. Pat. No. 7,174,065; GB2,350,961A; GB2,456,170A; US2006/0067611 “Wavelength Selective Reconfigurable Optical Cross-connect”; and WO2007/131649, “Method Of Forming An Image And Image Projection Device”, which describes a method of forming an image comprising providing a device for imparting respective phase-shifts to different regions of an incident wavefront, wherein the phase shifts give rise to an image in a replay field, and causing zero-order light to be focused into a region between the replay field and the device. Further background can be found in “Aberration correction in an adaptive free-space optical interconnect with an error diffusion algorithm”, D. Gil-Leyva, B. Robertson, T. D. Wilkinson, C. J. Henderson, in 1 Jun. 2006/Vol. 45, No. 16/APPLIED OPTICS pp. 3782; in A. A. Neil, M. J. Booth, and T. Wilson, “New modal wavefront sensor: a theoretical analysis”, J. Opt. Soc. Am. A, 17, 1098 (2000); in “Holographic wave front optimisation on an adaptive optical free-space interconnect”, Chapter 7, University of Cambridge PhD dissertation (http://search.lib.cam.ac.uk/?itemid=|depfacaedb|565372), June 2005, by Gil Leyva Zambada D.; and in W. A. Crossland, I. G. Manolis, M. M. Redmond, K. L. Tan, T. D. Wilkinson, M. J. Holmes, T. R. Parker, H. H. Chu, J. Croucher, V. A. Handerek, S. T. Warr, B. Robertson, I. G. Bonas, R. Franklin, C. Stace, H. J. White, R. A. Woolley, and G. Henshall, “Holographic optical switching: the ROSES demonstrator”, J. Lightwave Techn. 18, 1845 (2000).
The grating patterns previously described, used to deflect an optical input beam to output fibres in a LCOS-based switch, have drawbacks arising from insertion loss and crosstalk, due to the limitations in fabricating a perfect SLM and in displaying an ideal phase pattern. This results in significant power being diffracted into unwanted diffraction orders. This is in part due to the non-ideal response of liquid crystal material, and this particular issue is addressed in our co-pending UK patent application no. 1102715.8 filed 16 Feb. 2011.
We now describe techniques which aim to mitigate crosstalk, based on a technique which we will refer to as wavefront encoding.